A census of the Carina Nebula - II. Energy budget and global properties of the nebulosity

Abstract

The first paper in this series took a direct census of energy input from the known OB stars in the Carina Nebula, and in this paper we study the global properties of the surrounding nebulosity. This detailed comparison may prove useful for interpreting observations of extragalactic giant H II regions and ultraluminous infrared (IR) galaxies. We find that the total IR luminosity of Carina is about 1.2 x 10 7 L ⊙ , accounting for only about 50-60 per cent of the known stellar luminosity from Paper I. Similarly, the ionizing photon luminosity derived from the integrated radio continuum is about 7 x 10 50 s -1 , accounting for ∼75 per cent of the expected Lyman continuum from known OB stars. The total kinetic energy of the nebula is about 8 x 10 51 erg, or ∼30 per cent of the mechanical energy from stellar winds over the lifetime of the nebula, so there is no need to invoke a supernova (SN) explosion based on energetics. Warm dust grains residing in the HII region interior dominate emission at 10-30 μm, but cooler grains at 30-40 K dominate the IR luminosity and indicate a likely gas mass of ∼10 6 M ⊙ . We find an excellent correlation between the radio continuum and 20-25 μm emission, consistent with the idea that the ∼80-K grain population is heated by trapped Lyα photons. Similarly, we find a near perfect correlation between the far-IR optical depth map of cool grains and 8.6-μm hydrocarbon emission, indicating that most of the nebular mass resides as atomic gas in photodissociation regions and not in dense molecular clouds. Synchronized star formation around the periphery of Carina provides a strong case that star formation here was indeed triggered by stellar winds and ultraviolet radiation. This second generation appears to involve a cascade toward preferentially intermediate- and low-mass stars, but this may soon change when η Carinae and its siblings explode. If the current reservoir of atomic and molecular gas can be tapped at that time, massive star formation may be rejuvenated around the periphery of Carina much as if it were a young version of Gould's Belt. Furthermore, when these multiple SNe occur, the triggered second generation will be pelted repeatedly with SN ejecta bearing short-lived radioactive nuclides. Carina may therefore represent the most observable analogue to the cradle of our own Solar system.